1. Field of the Invention
In general, the present invention relates to waste water treatment facilities where solids are separated out of waste water. More particularly, the present invention relates to storage tank configurations and processes used within storage tanks to separate solids from waste water.
2. Prior Art Description
In most modern industrialized countries, waste water is collected and treated prior to being released back into the environment. Waste water is the general name given to the water that flows down drains and into the local sewer system. Waste water also comes from many factories that generate waste water as part of an industrial manufacturing process. Waste water in the local sewer system is typically treated by the local municipality. Waste water generated by factories is often treated on-site by a private water treatment plant.
Waste water contains many contaminants. As such, the waste water must be treated before it is returned to the environment. Typically, the first step used to treat waste water is to separate solid materials out of the waste water. Waste water contains many solids. Runoff from street sewer grates contain leaves, sticks, garbage and a variety of other debris. Waste water from buildings contains fecal matter and paper.
In the prior art, sewers guide waste water to a municipal waste water treatment facility. In this facility, the waste water is first sent into settlement tanks. In a settlement tank, waste water is allowed to come to rest. Any solid matter that is heavier than water will collect at the bottom of the settlement tank. Any solid matter that is lighter than water will float to the top of the settlement tank. The solids floating at the top of the settlement tank are skimmed away. The solids at the bottom of the tank are much harder to separate and collect from the liquid stream.
Solids are typically not drawn up through the water. To do so would mix the solids back into the remaining waste water. Rather, solids at the bottom of a settlement tack are usually pushed to one point in the tank by a slow moving conveyor. An outlet port is present at the bottom of the tank. Once enough solids have been collected, the outlet port is opened. Once opened, the pressure head of the waste water pushes the collected solids out of the outlet port. The discharged solids and waste water are collected in another tank. The discharged waste water has a concentration of solids of approximately 1%-2%, which is still at least 98% water.
Prior art system that use conveyors in settlement tanks to move solids near a backwash port are exemplified by U.S. Pat. No. 3,857,788 to Smith, entitled Sediment Apparatus With Plural Sludge Discharge Feature.
Once the sludge heavy backwash water is removed from the settlement tank, the sludge heavy backwash water is sent to a dewatering facility. The dewatering facility removes water from the solid waste until the remaining solids reach the consistency of sludge, which is typically between 15% and 30% solids. The sludge can then be incinerated, buried or land applied to increase the fertility of soil.
Facilities that dewater sludge-heavy waste water are exemplified by U.S. Pat. No. 4,160,732 to Maffet, entitled Mechanical Dewatering Process.
One of the main problems with the prior art treatment of sludge-heavy water is that dewatering facilities require a great deal of energy to run. Energy is expensive and is becoming more expensive year after year. Furthermore, dewatering facilities also consume large areas of space in a waste water facility.
A need therefore exists for a new system and method that increases the efficiency at which sludge is separated from water in the original settlement tank. By creating sludge-heavy backwash that has a higher concentration of solids, less dewatering is required. Less dewatering results in less use of energy, less processing spacing, and greater cost savings. This need is met by the present invention as described and claimed below.